(i) Field of the Invention
The present invention relates to an information storage apparatus and an information reproducing method for reproducing information recorded on an information storage medium.
(ii) Description of the Related Art
As a high-density information storage medium for recording and reproducing sound data or image data, an optical storage medium or a magnetic storage medium is known. Spiral or concentric tracks are typically provided on such an information storage medium, and these tracks are divided into a plurality of regions which are generally called sectors. Also, there is known an information storage apparatus which records information by writing marks in these sectors by a magnetic field or heat or an information storage apparatus which reproduces information by reading the marks by the magnetic field or heat.
With recent improvement in the computer technology, a data size or a quantity of sound data or image data to be used is increasing and enhancement of recording density of the information storage medium is eagerly desired. Narrowing the track pitch is demanded in order to greatly improve the recording density of the information storage medium.
As a technique for narrowing the track pitch, there is proposed one called land and groove recording by which information is recorded on both flute-like grooves and protruding lands alternately provided on the storage medium.
FIG. 1 is a view showing sectors on an information storage medium adopting the land and groove recording.
FIG. 1 illustrates three grooves 1, 2 and 3 and two lands 4 and 5 alternately provided with respect to these grooves 1, 2 and 3. Both the grooves 1, 2 and 3 and the lands 4 and 5 are used as tracks, and a plurality of sectors are provided to each track. Numbers for differentiating the sectors are given to these multiple sectors. For example, the -10th sector Sct-10, the -ninth sector Sct-9, the -eighth sector Sct-8, . . . , the 10th sector Sct10, the 11th sector Sct11, the 12th sector Sct12, . . . , the 30th sector Sct30, the 31st sector Sct31, the 32nd sector Sct32, . . . are provided on the three grooves 1, 2 and 3 in the drawing, and the 0th sector Sct0, the first sector Sct1, the second sector Sct2, . . . , the 20th sector Sct20, the 21st sector Sct21, the 22nd sector Sct22, . . . are provided to the respective two lands 4 and 5. That is, serial numbers are given to respective sectors provided in one track, and numbers given to a sector are different by 10 from those given to another sector adjacent to the former in a transverse direction of the tracks. Further, the number given to the sector is increased toward the central direction (inner direction) of the information storage medium and decreased toward the outer peripheral surface (outer direction).
In this manner, both the grooves and the lands are used as tracks in the land and groove recording. Therefore, the track pitch in a technique such that only lands are used as tracks is, e.g., 0.9 μm, whereas the track pitch in the land and groove recoding is an extremely narrow pitch, i.e., 0.65 μm. The recording density can be greatly increased if the linear recording density remains unchanged. Accordingly, the above-described technique is very important for realizing the high-density recording.
However, if the land and the groove recording and the like is adopted to greatly narrow the track pitch, when reading a mark on a given track, cross talk caused due to a mark on a track adjacent to that track prevents the mark from being read. For example, when reading the mark recorded in the 11th sector Sct11 in FIG. 1, cross talk occurs due to the mark in the first sector Sct1 or the mark in the 21st sector Sct21.
FIG. 2 is a graph showing an example of cross talk.
The upper part in FIG. 2 shows a signal waveform of a read signal obtained when performing reading with respect to a sector in erase state having no mark therein. A mark is written in a sector adjacent to the sector in erase state in a transverse direction of the tracks.
Further, the lower part of the FIG. 2 graph shows a gate signal indicative of a significant part in the read signal. In the signal waveform of the read signal shown in the upper part of the graph, only a portion corresponding to a time interval during which the waveform of the gate signal shown in the lower part rises is a significant signal waveform.
A flat waveform and a spike-like waveform exist in the signal waveform of the read signal, and the flat waveform is indicative of a signal caused due to a sector in erase state and the spike-like waveform is indicative of cross talk owing to a sector on an adjacent track. The signal intensity of such cross talk may be strong such that the signal is hardly differentiated from an original read signal. In such a case, a mark in a sector as a read target is prevented from being read.
Although the above-described problem prominently occurs in an optical disk device adopting the land and groove recording in particular, it occurs not only in such a device but it is typically generated in an information storage apparatus for reproducing information on an information storage medium having a narrow track pitch.